This invention pertains generally to airborne radar systems and more particularly to a method for enabling the receiver in such a system to be used to measure the range of a signal source.
It is known that the range of a signal source from an airborne radar may be determined with some degree of accuracy by measuring the differential time delay between signals traveling on a direct path between the airborne radar and the signal source and signals traveling on an indirect path (ground reflected) between the airborne radar and the signal source and the angle between the direct and indirect paths at such radar. Although it is relatively easy to apply known correlation techniques to measure the differential time delay, it is difficult to measure the angle between the direct and indirect signal paths. The difficulty derives from the fact that that angle is, if the signal source is at an appreciable range, relatively small and any error in measurement has an inordinate effect on the accuracy of the determined range.
It is also known that the range of a signal source may be determined by applying the well-known technique of triangulation. Thus, with the distance between two aircraft serving as a base for triangulation, azimuthal angles between each aircraft and the signal source may be measured to provide the necessary information to determine the range between the signal source and either aircraft. However, triangulation is not effective when more than one signal source is present because so-called "phantom" responses then exist so there is no way to be sure that the azimuthal angles are referenced to the same signal source. Further, even when there is a single signal source, triangulation does not uniquely define the position of an airborne signal source; all that is defined is a line in space corresponding to the intersection between the two vertical planes defined by the two azimuthal measurements. The signal source then is located somewhere on such line in space.
Another known way of passively measuring range between an airborne radar and a signal source requires that radars on two aircraft be operated in a correlation mode to measure the time delay (range difference) between the direct path signals received by each aircraft. In order to implement such a system, each of the two aircraft must also be equipped with a wideband communication link capable of relaying direct path signals between aircraft.